The present invention relates Lo a thermoplastic fluorine-containing resin composition useful for calendering in which the composition is thinned into a sheet or film.
Fluororesins are widely used in the form of sheet, film and laminate in various fields by reason of their characteristic properties such as good heat resistance, good chemical resistance and good weather resistance. Thermoplastic fluororesins in the form of sheet and film are employed in various uses by reason of their heat melt characteristics. Such thermoplastic fluororesins are produced in the form of sheet and film, mainly by extrusion. As well as extrusion, calendering is known as a method for continuously producing materials in the form of sheet or film. In calendering, material is melted by and passed between heated rollers to be thinned into a sheet or film. The thickness of the thinned product is adjusted by changing the space between the rollers. In calendering, productivity per unit time and yield are respectively high. Therefore, calendering is widely used for producing rubber, vinyl chloride resin and the like in the form of sheet or film. In contrast, calendering is not used for producing thermoplastic fluororesins in the form of sheet or film. In general, it is necessary that resin used in calendering has an elasticity as that of rubber and is easily removed from a calender roller and sufficiently high is thermoplasticity to be rolled by external force. In calendering, resin is heated by a large shear force from two rollers. Thus, when a resin such as polystyrene or polyethylene is calendered, this resin is drastically lowered in viscosity by the heating. With this, adhesion of the resin to the rollers substantially increases, and thus it becomes difficult to remove the resin in the form of sheet or film from the rollers. Thus, such resin is not suitable for calendering.
Calendering is a useful method for producing a composite film material, too. This composite material is produced at first by calendering a material into a film, then by placing this film on a cloth (e.g., polyester cloth or glass cloth) coated with an adhesive, and then by adding a pressure to bond together the film and the cloth. It is known to use vinyl chloride resin as a material of the film.
Conventional thermoplastic fluororesins are generally high in crystallinity, and thus many of them decrease drastically in melt viscosity at temperatures at or higher than their melting points. Furthermore, they do not have a sufficient elasticity to be removed from calender rollers. Therefore, conventional thermoplastic fluororesins are not suitable for calendering. In contrast, fluorine-containing elastomeric polymer which is called fluororubber is a material usable in calendering as well as other common rubbers. However, fluorine-containing elastomeric polymer itself does not have a particular melting point, and is relatively high in melt viscosity when heated as compared with thermoplastic resins. Therefore, the calendered elastomeric polymer in the form of sheet or film does not have a good melt characteristic.